It already does that. Look at the sketch. When ArduinoISP gets the "Q" message for AVRDUDE indicating that programming is done it sets the four interface pins to INPUT and turns off the PMODE LED (Pin 7).

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jihelbi

Searching furthermore I discovered the issue. I use a standalone 328P and I emulate the led on Arduino UNO pin 13 by a simple 4.7 K resistor instead of the OP on Uno rev 3. Since my IR decoder was connected by a 4.7 K to the pin on the target (to avoid the need to disconnect IR decoder for programming) the input is on a dividor bridge.